Marcesence

The word for today: MARCESENCE (marh-CESS-ent). A suitable word, as fall is fully upon us now. Trees have developed abscission layers; cells which act as a tourniquet, pinching off the xylem and phloem, allowing the plant to resorb the chlorophyll. This is the process which lets the brilliant fall pigments become visible. It is also the process which causes leaves to fall. Its a metabolically conservative process -- maintaining leaves requires energy. There simply aren't enough resources in the winter months to support a full canopy of leaves, and the freezing temperatures would result in cell lysis (bursting), as water expands as it freezes.

All this is logical, but why do we see trees with flags of dead leaves on them long into the winter months? This is where marcesence comes in. In marcesent trees, the abscission layer is functioning, as the leaves turn brown and die. The layer, however, never completes its task -- full seperation never occurs. This is often seen as a juvenile characteristic, occuing mainly in young, or young portions of trees. Oaks, Beeches, and Ironwood are the species most affected.

Exactly why this occurs is still unknown. Sometimes, a frost will occur early, before the abcission layer has formed, resulting in retained flags of leaves. Often this is not case. Many hypothesize that the retention of leaves is a protective measure. Herbivores, such as moose and deer feed on low, young twigs through the winter months. When leaves are retained, these twigs become far less palatible.

The big, the old, and the ugly

Gymnosperm make up a very diverse group of organisms-- so big you can drive your car through them, old enough to be your great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-greatgrandparent, and come in nearly every shape and size.


The Giant Redwoods of California can reach heights upwards of 300 feet, and can live up to 3000 years. Interestingly enough, this monstrousity of a tree produces the smallest of cones, only 4-7 cm in length. As many as 11,000 cones can be found on a tree at any one time.

Methusela the oldest living organism on this plant (to the best of our knowledge), is a statley Bristlecone pine. Based on ring counts, this plant is thought to be 4,789 years old. Rings are formed by the differences in growth rates between the summer and winter seasons. Summer wood is far lighter, as layers are rapidly laid down in response to ideal conditions. The darker rings are a result of the slower growth rate of the winter months. Rings of the Bristlecone pine are particulalry dense, as the conditions they must endure in the Rocky mountains in Nevada, Utah, and California can be quite extreme.

One of the most unique plants in existance has to be the welwitschia plant. Found only in the Namibia desert, this gymnosperm has but two leaves. These leaves, however, can grow up to two hundred feet long over the course of the plants 2000 year life span. During this time, the leaves tend to fray, giving it the appearance of many leaves. This living fossil won't be winning a beauty contest any time soon.

leaves on trees, or, how to predict rain

Humidity is a great predictor of upcoming rain. People are the only ones to be effected by increased moisture in the air, trees can feel it too. Unlike people, however, trees don't get frizzy hair -- instead, we see a curling of the leaves, especially those of Oaks and Maples. This is a mechanism adopted by the plant in its never ending battle to conserve water. The stomata of leaves are usually kept tightly shut during daylight hours, when water loss is more likely. On humid days, however, they'll open right up and take advantage of conditions. Further, the damp air softens the petiole, allowing the blade to be curled or flipped over more easily.